The present invention relates to the field of power supplies for light emitting diode based backlighting and more particularly to a controlled bleeder timed to absorb and or limit any power supply ringing.
Light emitting diodes (LEDs) and in particular high intensity and medium intensity LED strings are rapidly coming into wide use for lighting applications. LEDs with an overall high luminance are useful in a number of applications including backlighting for liquid crystal display (LCD) based monitors and televisions, collectively hereinafter referred to as a monitor. In a large LCD monitor typically the LEDs are supplied in one or more strings of serially connected LEDs, thus sharing a common current.
In order supply a white backlight for the monitor one of two basic techniques are commonly used. In a first technique one or more strings of “white” LEDs are utilized, the white LEDs typically comprising a blue LED with a phosphor which absorbs the blue light emitted by the LED and emits a white light. In a second technique one or more individual strings of colored LEDs are placed in proximity so that in combination their light is seen a white light. Often, two strings of green LEDs are utilized to balance one string each of red and blue LEDs.
In either of the two techniques, the strings of LEDs are in one embodiment located at one end or one side of the monitor, the light being diffused to appear behind the LCD by a diffuser. In another embodiment the LEDs are located directly behind the LCD, the light being diffused so as to avoid hot spots by a diffuser. In the case of colored LEDs, a further mixer is required, which may be part of the diffuser, to ensure that the light of the colored LEDs are not viewed separately, but are rather mixed to give a white light. The white point of the light is an important factor to control, and much effort in design and manufacturing is centered on the need for a correct white point.
Each of the colored LED strings is typically intensity controlled by both amplitude modulation (AM) and pulse width modulation (PWM) to achieve an overall fixed perceived luminance. AM is typically used to set the white point produced by the disparate colored LED strings by setting the constant current flow through the LED string to a value achieved as part of a white point calibration process. PWM is typically used to variably control the overall luminance, or brightness, of the monitor without affecting the white point balance. Thus the current, when pulsed on, is held constant to maintain the white point among the disparate colored LED strings, and the PWM duty cycle is controlled to dim or brighten the backlight by adjusting the average current. The PWM duty cycle of each color is further modified to maintain the white point, preferably responsive to a color sensor. It is to be noted that different colored LEDs age, or reduce their luminance as a function of current, at different rates and thus the PWM duty cycle of each color must be modified over time to maintain the white point.
Each of the disparate colored LED strings has a voltage requirement associated with the forward voltage drop of the constituent LEDs and the number of LEDs in the LED string. In the event that multiple LED strings of each color are used, the voltage drop across strings of the same color having the same number of LEDs per string may also vary, due to manufacturing tolerances and temperature differences. Ideally, separate power sources are supplied for each LED string, the power sources being adapted to adjust their voltage output to be in line with voltage drop across the associated LED string. Such a large plurality of power sources effectively minimizes excess power dissipation however the requirement for a large plurality of power sources is costly.
An alternative solution, which reduces the number of power sources required, is to supply a single power source for each color. Thus a plurality of LED strings of a single color is driven by a single power source, and the number of power sources required is reduced to the number of different colors, i.e. typically to 3. Unfortunately, since as indicated above different LED strings of the same color may exhibit different voltage drops, such a solution further requires an active element in series with each LED string to compensate for the difference among the respective voltage drops so as to ensure an essentially equal current through each of the LED strings of the same color. The LED string voltage drop is not a constant, as in particular the individual LED voltage drop changes as the LEDs age. Furthermore, the voltage drops of the LEDs of the LED strings are a function of temperature, and thus the voltage output of the power source must be set high enough so as to supply sufficient voltage over the operational life of the LED strings.
As explained above, each of the LED strings are pulse width modulated, i.e. the strings are individually switched between a conducting state in which the LEDs illuminate and a non-conducting stage in which the LEDs do not illuminate, and thus the power source experiences widely disparate rapidly changing demands. In a typical embodiment the LED strings are pulse width modulated via a simple FET which is turned on and off, and thus the current for each LED string is nearly instantaneously switched between the nominal LED string current and zero current. Ideally, the power source used should have a high frequency response, and thus be capable of supporting the rapidly changing load, but unfortunately this is costly. The power source may be constituted of a switching power source, having therein a PWM component independent of the PWM control of the LED string.
FIG. 1 illustrates a high level schematic diagram of an LED backlighting system 10 comprising: a power source, illustrated as switching power supply 20 associated with a single LED string 30 connected in series with an FET 40 and a sense resistor 50. FET 40 is switched from a conducting state to a non-conducting state thereby pulse width modulating LED string 30. FIG. 2A illustrates the desired output current of power supply 20, with the y-axis representing current drawn from power supply 20 and the x-axis representing time. Power supply 20 experiences a load condition 100 when FET 40 is in a conducting state and a no-load condition 90 when FET 40 is a non-conducting state. The output current of power supply 20 thus exhibits a near zero current during the no-load condition 90 when FET 40 is in a non-conducting state, and a high output during load condition 100 when FET 40 is in a conducting state thereby enabling a current draw by LED string 30.
FIG. 2B illustrates the output voltage of power supply 20 in the event that power supply 20 does not exhibit a sufficiently high frequency response, with the y-axis representing output voltage of power supply 20 and the x-axis representing time. The x-axis is in 1:1 correspondence with the x-axis of FIG. 2A. The voltage output of power supply 20 exhibits a ringing component 120 with consequent overshoot and undershoot at the beginning of the load condition 100, and similarly exhibits ringing 130 at the beginning of the no-load condition 90. FIG. 2C illustrates the resultant output voltage of power supply 20 associated with the waveform shown in FIG. 2B. FIG. 2D illustrates the resultant output current of power supply 20 associated with the waveform shown in FIG. 2B. The output current of power supply 20 exhibits a ringing component 140 including overshoot and undershoot at the beginning of load condition 100.
Since for each power supply there are typically a plurality of LED strings, ideally the PWM cycle for each of the LED strings is distributed so that the power supply does not experience a no-load condition, and therefore the ringing is minimized. Unfortunately, this puts an additional limitation on the PWM timing control, and may not always achievable. To the extent a current overshoot may occur, the overshoot must be taken into account in specifying the power supply. Furthermore, in the event that the control for the LED strings samples the amount of current flowing through each LED string, care must be taken to ensure that a sample is not obtained during the ringing, as such a sample may not be representative of the actual current flow.
What is needed, and not provided by the prior art, is a means for controlling or limiting the ringing associated with a power supply experiencing near instantaneous changes in current flow.